All the displays are basically web views running scripts like a browser does, including calls to the SDK.
An analog gauge shows one value. Glass cockpits can show and/or evaluate hundreds and thousands of parameters, having multiple pages etc.
So one display needs to evaluate all the SDK calls, render the web view (also done by the CPU) and then let the GPU render in the model which is the least amount of work.
I wonder how you compare a simple thing with a very complex one and then expect the same performance impact.
As for the Arrow: I guess it’s simply not well coded, since you have to take care about how often you update the screen and mostly static displays could very well skip many frames without you really noticing it (that’s basically what the glass cockpit refresh rate does).
If the sim gets told to evaluate the data on every frame, then it will do so. Prohibiting it would mean to cut down possibly necessary updates for some addons.
Of course there are many locations to optimize performance, in 3rd party addons as well as in the sim. And I’m sure that’s going to happen. But asking a question in a rhetorical sense is a bit strange since you don’t seem to see the amount of calculations happening behind such a display.